The present invention relates in general to lasers having a solid-state gain-region in the form of planar-sided slab of a gain-material or gain-medium. The invention relates in particular to a solid-state laser including a resonator having a gain-region including two or more prismatic slabs of a gain-medium, the slabs are cooperatively arranged such that light circulating in the resonator follows an extended convoluted path through the prismatic slabs.
Solid-state lasers having a gain-region in the form of a slab of gain-medium are generally referred to as slab lasers. The slab-like form of the gain-medium is particularly suited for optical pumping by one or more diode-laser arrays. In one common form, the gain-medium is in the form of an elongated parallelepiped or rhomb. Pump-light from one or more diode-laser arrays is directed through lateral faces of the gain-medium. The gain-medium is arranged in a laser-resonator, and end-faces thereof are configured such that laser-light circulating in the resonator follows a zigzag path through the gain-medium. Examples of this type of arrangement are disclosed in U.S. Pat. Nos. 5,235,610; 5,748,664; and 5,872,804. One shortcoming of these, and other zigzag arrangements, is that a significantly larger volume of the gain-medium is optically pumped than the volume of the gain-medium from which laser-light is extracted. This can present a problem in causing excessive heating of the gain-medium and waste of pump-light power. This especially true in three-level systems where the excess of pumped gain-medium behaves as an absorbing aperture.
This particular problem is avoided by an alternative arrangement, disclosed in U.S. Pat. No. 5,249,196. Here a gain-medium is in the form of a pentaprism. One of two faces of the pentaprism subtending a ninety-degree angle is used as a end-reflector of a laser-resonator. An external mirror, facing the other of the two ninety-degree faces provides the other end-reflector of the laser-resonator and is used as an output-coupling mirror. Diode-laser pumping is arranged from multiple diode-laser sources arranged around three reflecting faces of the pentaprism such that pump-light is directed into the pentaprism along all (three) legs of a folded path followed by laser-light circulating within the pentaprism, but only along those legs.
A problem common to both the above-described zigzag and pentaprism arrangements of gain-media and associated resonators is that only a relatively small proportion of the volume of a gain-medium is traversed by laser-light circulating in the resonators. This proportion is often referred to by practitioners of the art as a xe2x80x9cfill-factorxe2x80x9d. In such arrangements, scaleability to higher laser power may be limited by the availability of boules or blanks of gain-medium of sufficient size and optical quality.
One solution to this fill-factor problem is proposed in a paper xe2x80x9cAlternating-Precessive-Slab Preamplifierxe2x80x9d, NASA Tech Briefs, July 1997, page 52. Here a slab of gain-medium for a laser preamplifier has a basically square shape with two adjacent corners thereof truncated at forty-five degrees to lateral faces of the square slab. One of the truncated faces serves as an entrance face and the other as an exit face. Light entering the entrance face at normal incidence thereto undergoes a succession of 90 degree internal reflections from the lateral faces of the slab and eventually leaves the slab via the truncated exit face at normal incidence thereto. The path of light in the slab can be described as a convoluted path having a plurality of parallel longitudinal and transverse legs orthogonal to each other and at forty-five degrees to lateral faces of the slab.
A disadvantage of this arrangement, and many other slab arrangements, is the angular relationship of all faces of the slab with each other must be controlled with relatively high precision. This adds considerably to be difficulty and expense of making such slabs. There is a need for a slab laser gain-medium arrangement which offers a high fill factor, extensive scaleability without a requirement for expensive gain-medium components.
In one general aspect, a slab laser in accordance with the present invention includes a laser-resonator and a plurality of generally-triangular prismatic slabs of a solid-state gain-medium located in the resonator. The prismatic slabs are cooperatively arranged such that laser-light circulating in the laser-resonator follows a convoluted path through the prismatic slabs. Preferably each of the prismatic slabs changes the direction of the convoluted path at least once by about one-hundred-eighty degrees (180xc2x0).
In one particular aspect, a slab laser in accordance with the present invention comprises a laser resonator having a gain-region located therein. The gain-region includes at least two prismatic slabs including a solid-state gain-medium. Each of the prismatic slabs has two reflecting faces subtending an angle of ninety degrees and at least a first entrance-exit face. The first entrance-exit face faces the reflecting faces within the angular subtense thereof and is inclined at forty-five degrees thereto. The entrance-exit face has a length longer than the length of the longest of the reflecting faces. The prismatic portions are cooperatively arranged such that laser-light circulating in the laser-resonator follows a first convoluted path through the gain-region. The first convoluted path includes a plurality of spaced-apart parallel longitudinal legs, adjacent ones thereof being connected by a corresponding transverse leg at ninety degrees thereto. The longitudinal and transverse legs are inclined at forty-five degrees to the reflecting faces.
In one embodiment of the inventive laser, the laser-resonator is terminated by first and second mirrors and the laser-light circulates in the laser-resonator between the first and second mirrors. The first and second mirrors are arranged such that the circulating laser-light follows the first convoluted path in opposite directions of travel between the first and second mirrors.
In another embodiment of the inventive laser the laser-resonator is terminated by first and second mirrors and the laser-light circulating in the laser-resonator circulates between the first and second mirrors. A prism is positioned at a mid-point in the laser-resonator to reflect the circulating laser-light. The prism has first and second reflective surfaces at an angle of forty-five degrees to each other. The first and second mirrors and the prism are arranged such that the circulating laser-light follows the first convoluted path through the gain-region in travelling from the first mirror to the prism, and follows a second convoluted path through the gain-region in traveling from the prism to the second mirror. The second convoluted path is spaced apart from and parallel to the first convoluted path.
In yet another embodiment of the inventive laser the laser-resonator is terminated by a mirror and prism having first and second reflective faces at an angle of forty-five degrees to each other. The mirror and the prism are arranged such that the circulating laser-light follows the first convoluted path in traveling from the mirror to the prism, and a second such convoluted path in traveling from the prism to the mirror. The first and second convoluted paths are overlapping and parallel to each other.
In all preferred embodiments of the inventive laser, the prismatic slabs are optically-pumped by pump-light optically delivered from a diode-laser array through one of the reflecting-faces thereof. Optical delivery may take place via a cylindrical lens for reducing the divergence of light from the diode-laser array in a fast-axis thereof. Optical delivery may also take place via a tapered optical-waveguide for concentrating light from a diode-laser array having emitting-aperture dimensions larger than the reflecting-face. One or more of the prismatic slabs may be clad on upper and lower opposite lateral faces thereof with a material transparent to the laser-light and the pump-light, and having a lower refractive index than the refractive index of the gain-medium.
A preferred general form of the prismatic gain-medium slabs may described as being generally-triangular. The terminology generally-triangular is intended to cover a true (three-cornered) isosceles triangular form wherein two reflecting faces of equal length connect to form what may be described as an apex-corner of the slab, and the two reflecting faces connect with the first entrance-exit or hypotenuse face to form what may be described as hypotenuse-corners of the slab. The terminology generally-triangular is also intended to cover prismatic slabs having the apex-corner truncated to form a second entrance-exit face between the reflecting faces and parallel to the first entrance-exit face, and additionally or separately, having one or both of the hypotenuse-corners truncated.
The prismatic slab form of the gain-medium, which is a feature of all inventive lasers described herein is particularly suitable for cooling the gain-medium by conduction through lateral faces of the slab. This allows cooling to cryogenic temperatures. This provides a particular benefit in using quasi three-level gain-medium systems, the performance of which improves with decreasing temperature. Such systems include the gain-media Yb:YAG, Tm:YAG, Tm:Ho:YAG, CTH-YAG, Ho:YLF, Tm:YLF, and Tm:YALO.
Using two or more prismatic gain-medium slabs has cost and manufacturing advantages over prior art slab lasers including a single monolithic slab. This results from fabrication and alignment tolerances of the prismatic slabs of gain-medium being generally looser and material costs generally less than for a single monolithic slab providing an equivalent path-length through the gain-medium. Cost advantages realized will be dependent, inter alia, on the size of the slab and the particular gain-medium.